Mirror substrates for use in projection optics systems within extreme ultraviolet (EUV) lithography (EUVL) scanners must meet stringent thermal expansion requirements in order to maintain their intended surface shape (known as “figure”) upon temperature changes caused by normal operation cycles of the scanner. For this reason, the preferred material for manufacturing state of the art projection optics mirror substrates is Ultra Low Expansion glass (ULE® Glass), manufactured by Corning Incorporated. Glass sold by Corning Inc. under the glass code 7973 is specifically tuned for EUVL applications, and is characterized by high degrees of precision and accuracy, which permit supplying glass with properties that are narrowly targeted to each specific application.
As the EUVL source power increases to meet the requirements of a high volume manufacturing system, the thermal expansion specifications for the projection optics mirror substrates are becoming tighter. For example, the maximum allowable CTE (coefficient of thermal expansion) slope, the rate of change of CTE with temperature, which is 1.6 ppb/K2 at 20° C. for standard ULE® is expected to decrease below 1.0 ppb/K2. Taking full advantage of the better performance enabled by low slope materials requires improved material uniformity and more precise knowledge of the Tzc (zero crossover temperature), the temperature at which the CTE is equal to zero.
Tzc in ULE® glass is controlled by composition and by the thermal history of the glass. Qualification of production ULE® glass to ensure that it fulfills specification requirements for Tzc involves measurements of CTE using an indirect acoustic method. The method has been successful so far, but it has some shortcomings. It relies on the material having a well-defined thermal history. Measuring material with different thermal history requires the calibration to be corrected for the specific thermal history. There is potential for uncontrolled factors, such as hydroxyl (—OH) content, affecting the calibration and going unnoticed, which would introduce errors in the Tzc calculated for the part. Efforts to correlate the technique to absolute dilatometry show a residual error in the order of 1 to 2° C. in the zero crossover temperature calculated for the parts. Due to its indirect nature, and its reliance on an empirical calibration, makers of EUVL scanners are uncomfortable relying on its results for qualifying material when requirements for Tzc accuracy are in the order of a few degrees C.
On the other hand, the value of Tzc can be ascertained by measuring a sample of glass in an absolute dilatometer, including a Fabry-Perot interferometer (FPI). Absolute FPI dilatometry is a well established technique, but it is not suitable for controlling glass in a production environment. It requires carefully finished samples, which are expensive and take a long time to manufacture (4 to 8 weeks). It requires expensive specialized equipment and highly skilled personnel. It is potentially affected by subtle and hard-to-quantify effects such as temperature dependence of reflection coatings, and the quality of optically contacted bonds. A high resolution measurement requires use of “end caps” preferably made of the same material under evaluation. These ends caps are required to be transparent, meaning that only optically clear materials can be measured at the highest resolution. Due to the relatively large size of the needed samples (100 grams or more), it is sometimes difficult to select a sample that truly represents the material in a part. It is slow, typically taking a week or more to measure a sample. The slow speed, together with the complexity of measurement setup and sample requirements, makes this technique very expensive and severely restricts its use.
Badami and Patterson proposed methods for highly accurate measurements of dimensional changes in commonly-owned U.S. Pat. Nos. 7,239,397 and 7,426,039. These methods are able to measure dimensional changes with high precision on monolithic samples. However, the instruments are highly specialized and complex, and it was not demonstrated that they can reliably measure CTE(T) with the required precision and accuracy.
A highly complex Optical Heterodyne Interferometric Dilatometer for determination of absolute CTE of EUVL materials has been demonstrated by Takeicha et al. (Proceedings SPIE vol. 5751, p 1069 (2005) and vol. 6151 p 61511Z-2 (2006). The apparatus relies on expensive components such as a frequency-stabilized laser. The method relies on mechanical contact between a sample and a reference surface, subjecting the measurement to potential errors that are difficult to quantify. Furthermore, it was not demonstrated that the technique is capable of measuring the thermal expansion behavior with the required accuracy over a wide temperature range.
Various other methods involve mechanical contact between one end of the sample and a reference surface, introducing a source of uncertainty that is hard to quantify.
The photoelastic sandwich seal technique can be used to measure the difference in CTE between samples of two materials using much simpler and faster equipment than absolute dilatometry, but it has shortcomings of its own. It also requires relatively expensive and carefully made samples, with a long lead time. It measures differences in CTE between two materials, and not directly the absolute CTE or Tzc. Establishing absolute Tzc requires correlation to a reference. Its ultimate resolution in establishing the absolute CTE of a sample is 3 to 5 times less accurate than needed for high-end applications such as critical mirrors in EUVL projection systems. Due to these reasons, the technique is not well suited for direct Tzc characterization in a production environment.
Interferometry has been shown to be useful in providing maps of variations of CTE, and thus Tzc in ULE® glass. Refractive index variations correlate to TiO2 concentration variations, which in turn correlate to CTE. Interferometry maps exhibit the highest resolution of any technique both spatially and in CTE (in the range of parts per trillion), but do not provide information about the absolute values of the CTE.
A photoelastic method for determining the zero crossover of the CTE(T) curve in ULE® glass was proposed by the present inventor in commonly-owned U.S. Pat. No. 8,328,417. That method has the distinct advantage of not requiring an optically contacted sample or complex and expensive laser systems, but it only aims to measure the zero crossover temperature, and can only marginally give information on the temperature dependence of CTE(T). Further, the samples needed for the measurement are bulkier than required by the present invention, and thus require longer stabilization times.
Thus, there is an unmet need for a technique that allows quick and inexpensive measurement of the absolute CTE(T) of a small sample of ULE® glass, without the need for expensive equipment or samples that have high cost and take a long time to manufacture. Furthermore, such an invention could be used in production to provide an absolute reference for relative index measurements carried out using interferometry, which would allow this higher resolution technique to replace highly labor intensive, lower spatial resolution ultrasonic velocity measurements.
In a broader context, measurements of the coefficient of thermal expansion for ultra-low expansion material are difficult to carry out, since they involve measuring displacements on the nanometer (nm) scale on macroscopic samples measuring in the order of 10 mm or more. This level of performance cannot be achieved using commercial dilatometers, which in most cases involve mechanical contact between a sample and a mechanical sensor or a reference surface. Even high-quality commercial dilatometers based on optical interference measurement principles, such as instruments manufactured by Rigaku and Linseis, are susceptible to thermally induced drifts in the order of tens of nanometers, rendering them of little value for evaluating ultralow expansion materials with CTE values on the order of 10−9/° C. (1 ppb/K).